The two fold purpose of marriage is the salvation and good of the spouses and the bringing forth of new life:
"By its very nature it [marriage] is ordered to the good of the couple, as well as to the generation and education of children. " CCC 1660 (emphasis added)
"Unity, indissolubility, and openness to fertility are essential to marriage." CCC 1664 (emphasis added).
So the first purpose of marriage, the primary purpose, is the good of the spouses. The marriage comes first. What fruit will come from that marriage will be up to God, the author of all life. Not every marriage brings forth children, even when they are fully open to it. The marriage is a a sacramental one just the same.
Even a late in life marriage must be open to children--meaning, no artificial contraception, no continuous use of NFP with no openness to life. Meaning being open to God bringing other children (or service to children) into your lives together.
A late in life marriage is still called to be fruitful. That fruit just might not be biological children of their own.
For me, I struggle not with whether it is allows (of course, it is allowed) but whether marriage is worth it, if it is only for the my good, and not for the generation of life. For me, a marriage that will not include children has an inherent sense of selfishness for me. Built into it, there is a sense of indulgence I guess for me. IT is like the only reason then to get married is to be able to have licit sex (that is not even fruitful, so what is up with that?) And old people sex...ewwww! LOL
Not that I have not suffered greatly being on my own all these years, but I don't know, I guess I can still soldier on as I have been doing for decades, for some decades more.
I guess for me I wonder if a childless marriage is worthwhile--to me, to him, to society, to the Church. But I know it is allowed. Just don't know if it is worth it at this point.